The present invention relates to a drive-slip control system (ASR) for a motor vehicle in which a differential is provided for transmitting the engine output torque or gearbox output torque to two driven vehicle wheels. The ASR operates on the principle of retarding again a vehicle wheel having a tendency to spin and driven via the differential by actuating its wheel brake and, if a deceleration of both driven vehicle wheels becomes necessary, of reducing the engine output torque, e.g. by intervention in at least one of the ignition, the fuel supply, and throttle-valve position alteration.
A drive-slip control system is shown in German Offenlegungsschrift Nos. 1,806,671; German Offenlegungsschrift 3,127,302; DE 36 15 638 A1; and DE 38 10 449 A1. It is further more also known, as can be seen in DE 33 19 152 C2, to provide additional brakes, configured, for example, as multiplate brakes, for the driven vehicle wheels. The brakes form a structural unit with the differential but act on the driving side shafts. These known control system operating, at least in the initial phase of a situation requiring drive-slip control, with braking intervention have the disadvantage that, in cases in which the one driven vehicle wheel is rolling on an area of roadway which has a high coefficient of adhesion (.mu.) between the roadway and this wheel but the other driven vehicle wheel is rolling on an area of roadway which has a very low coefficient of adhesion, a considerable proportion of the driving torque provided by the engine via the gearbox has, as it were, to be dissipated to ensure that propulsive torque can be transmitted via the vehicle wheel which is more capable of transmission. The result of this is that, in the case of vehicles with a relatively low-powered engine, i.e. vehicles with an engine power of less than 70 kW, the weight of which is about 10000 N to 12000 N, their climbing ability in the above mentioned ".mu. split" conditions is no longer sufficient to allow negotiation of relatively steep gradients of, for example 20%. Hitherto, it has therefore only been possible to implement drive-slip control systems of the type mentioned at the outset on vehicles in the higher performance categories.
The use of lockable differentials with a controllable degree of locking for drive-slip control is shown in DE 35 28 389 A1; DE 37 08 063 A1; and DE 38 28 656 A1. It is possible with these differentials to compensate speed and hence also slip difference between the driven vehicle wheels, although it must then be possible, for the purpose of slip limitation, to adjust in a controlled manner the driving torque coupled into the drive train in terms of the absolute amount.
In order in this way to be able to implement an ASR which guarantees a sufficient dynamic stability of the vehicle, very high degrees of locking of the differential must be inputted, and this leads to considerable structural complexity as regards the locking elements which, in typical design, are in the form of friction clutches. Furthermore, a rapid torque reduction in the drive train must be possible but a torque reduction by altering the position of the throttle valve or the fuel feed, for example, would be too slow for a large number of situations because of the inertia of the engine and the drive train as a whole. Additional actuating devices are therefore required, e.g. a disc brake acting on the input shaft of the differential, or an actuator by which the engine clutch can be disengaged.
It is therefore an object of the invention to provide a drive-slip control system which is of simple overall construction and imparts good traction behavior, in particular good climbing ability, even in the case of lower performance vehicles.
This object is achieved according to the present invention by providing the differential with at least one locking device which develops, as the difference in the propulsive torques which can be transmitted via the driven vehicle wheels between the latter and the roadway increases, an increasing degree of locking which is, however, limited to a value of around 40% relative to the maximum amount.
By virtue of the constructional features of the present invention, namely, that the differential is provided with a locking device which, as the difference in the propulsive torques which can be transmitted via the driven vehicle wheels, between the latter and the roadway increases, develops an increasing degree of locking, but is limited as regards the maximum amount, it is ensured that only a markedly reduced part of the engine output torque is braked away by a braking intervention which may still be required because, due to the at least partial locking of the differential, a considerably higher proportion of the driving torque provided by the engine via the gearbox remains available, depending on the degree of locking which can be developed by the differential.
From the energy point of view, this is essentially due to the fact that no matter how the locking device is implemented, the amount of driving power which must be braked away to build up the degree of locking within the differential only corresponds to a torque difference, in contrast to a conventional drive-slip control system, in which, due to the braking intervention at the vehicle wheel with the tendency to spin, up to half of the driving torque provided by the engine has to be dissipated.
In order to achieve the required locking effect in the differential, at least one friction clutch is provided to bring about a frictional coupling of one of the driving ring gears or of one of the differential bevel gears of the differential to the cage on which these gears are rotatably mounted.
According to another feature of the drive-slip control system of the present invention, friction clutches are assigned individually to each of the side shafts of the driving axle. The friction clutches result in, as it were, an automatically operating locking device for which no control structure at all is required.
Alternatively or in combination with the foregoing features, electrohydraulic control elements are provided and are actuatable via solenoid valves which are controlled by output signals of an electronic control unit to produce control signals by processing output signals of wheel-speed sensors. The wheel speed sensor output signals, in their level or frequency, contain the information on the dynamic behavior at least of the driven vehicle wheels.
Likewise, as an alternative to or in combination with the above-described configurations of the drive-slip control system shear elements for the locking elements can be provided for the locking elements and filled with dilatant fluid, which likewise respond automatically and do not require any control apparatus.
Finally, another feature of the present invention is a configuration of the drive-slip control system which can be implemented at particularly low cost, for vehicles which are equipped with an anti-lock brake system which operates on the rear axle, i.e. the axle of the driven vehicle wheels, with common brake-pressure control.